1. Technical Field
The present disclosure relates generally to radio frequency (RF) signal circuitry, and more particularly, to second order harmonic cancellation for switches in RF front end circuits.
2. Related Art
Complex, multi-function electronic devices are comprised of many interconnected modules and components, each of which serves a dedicated purpose. As a general example, wireless communication devices may be comprised of a transmit chain and a receive chain, with the antenna and the transceiver circuit being a part of both the transmit chain and receive chain. The transmit chain may additionally include a power amplifier for increasing the output power of the generated RF signal from the transceiver, while the receive chain may include a low noise amplifier for boosting the weak received signal so that information can be accurately and reliably extracted therefrom.
The low noise amplifier and the power amplifier may together comprise a front end module or front end circuit, which also includes an RF switch circuit that selectively interconnects the power amplifier and the low noise amplifier to the antenna. The connection to the antenna is switched between the receive chain circuitry, i.e., the low noise amplifier and the receiver, and the transmit chain circuitry, i.e., the power amplifier and the transmitter. In time domain duplex communications systems where a single antenna is used for both transmission and reception, this switching between the receive chain and the transmit chain occurs rapidly many times throughout a typical communications session.
In the local area data networking context, WLAN or Wireless LAN, also commonly referred to as WiFi, as well as 802.11 (referring to the governing IEEE standard), is widely deployed. WLAN utilizes frequency allocations in the Industrial-Scientific-Medical (ISM) band, and specifically the 2.45 GHz range, also colloquially referred to as the 2 GHz band. More recent iterations of the IEEE WLAN standard also specify the use of the 5 GHz range in the ISM band, for which usage has been licensed. Another common local wireless data communications modality is Bluetooth, which is often utilized to interconnect peripheral devices. Because Bluetooth also utilizes the 2 GHz ISM band, the same antenna, as well as common blocks such as the oscillator circuit, bandgap reference, and power management units for WLAN signals may be shared. Accordingly, the 2 GHz antenna may be connected to a single pole, triple throw (SP3T) switch with a terminal for WLAN receive, another terminal for WLAN transmit, and a third terminal for Bluetooth signals (both transmit and receive). The 5 GHz antenna is exclusively utilized for WLAN transmit/receive, so a single pole, double throw (SPDT) switch is utilized.
An RF switch has several performance parameters, including insertion loss, isolation, return loss, and linearity. Insertion loss refers to the power lost in the RF switch, and is expressed in dB. It is defined by Pout−Pin (dB), where Pin is the input power applied to the RF switch, and Pout is the power at the output port of the RF switch. Isolation refers to the measure of signal attenuation, expressed in dB, between the active signal port and the inactive signal port. Return loss refers to the measure of input and/or output matching conditions, and is expressed in dB. Linearity, or power handling capability, is the capability of the RF switch to minimize distortion at high power output levels and is expressed in dBm. It is typically represented by the 1 dB compression point (P1 dB), or the point at which insertion loss is degraded by 1 dB. Harmonic distortion for a given output power level is expressed in dBc, or the dB below the carrier or fundamental frequency.
Generally, RF switches must generate as little harmonic distortion as possible. Governmental standards also restrict the output of spurious emissions including those from harmonic distortion to either −70 dBc or 43+10 log(P). Conventional front end circuits, including the RF switch, are fabricated on a bulk CMOS (complementary metal oxide semiconductor) substrate. However, there is a performance tradeoff between insertion loss and harmonic distortion under large signal operation. Furthermore, because of low mobility, low breakdown voltage, and high substrate conductivity associated with CMOS devices, an RF switch with low insertion loss, high isolation, wide bandwidth, and linearity is difficult to produce.
Stacked switches of series and shunt transistors may be utilized to sustain higher voltage swings, thereby improving the power handling and harmonics suppression characteristics. However, this is understood to result in a higher insertion loss because of the additional equivalent resistance of an “on” position. In one conventional implementation, a single switch may have an insertion loss of less than 0.8 dB, but with a second harmonic distortion of around −50 dBc and a third harmonic distortion of around −60 dBc at an output of 23 dBm. On the other hand, a triple-stacked switch may have an insertion loss greater than 1.3 dB, but with a much lower second harmonic distortion of around −63 dBc and a third harmonic distortion of around −83 dBc at an output of 23 dBm. Thus, insertion loss is higher than that of the stacked switches because of the resistance of three, as opposed to one transistor in the “on” state, and negatively impacts efficiency.
Therefore, there is a need in the art for an improved RF switch with harmonic suppression and low insertion loss.